The characteristics of the sound interaction with a bubble cloud produced
by a series of hypodermic needles are studied both experimentally and
theoretically. It has been observed that the sound-pressure level is magnified
when the incoming waves are at resonance with the first collective mode
(breathing mode) of the cloud. The magnification, as high as 20 dB, depends on
the void fraction. A simple theory based upon a forced mechanical oscillator is
developed. This theory suggests that when the incident wave frequency is close
to the fundamental frequency of the cloud, the system is at resonance. As a
result, a significant sound magnification occurs. The theory and the
experimental observations are in a good agreement. In this paper, the resonance
magnification is discussed in great detail and some practical applications of
this phenomenon are offered. [Work supported by the Office of Naval Research.]